Fix settings persistence, streaming stability, and UI polish

- Fix device settings partial update using model_fields_set for true merge
- Add missing interpolation_mode and smoothing to all API responses
- Fix send_pixels race condition when wled_client is None during stop
- Allow LED segments to exceed edge pixel count (float stepping)
- Fix modal scroll lock using position:fixed to prevent layout shift
- Show loading state for brightness slider until real value is fetched
- Remove stream description from stream selector dialog

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/routes.py

@@ -304,6 +304,7 @@ async def create_device(
                 border_width=device.settings.border_width,
                 interpolation_mode=device.settings.interpolation_mode,
                 brightness=device.settings.brightness,
+                smoothing=device.settings.smoothing,
                 state_check_interval=device.settings.state_check_interval,
             ),
             calibration=CalibrationSchema(**calibration_to_dict(device.calibration)),
@@ -339,7 +340,9 @@ async def list_devices(
                     display_index=device.settings.display_index,
                     fps=device.settings.fps,
                     border_width=device.settings.border_width,
+                    interpolation_mode=device.settings.interpolation_mode,
                     brightness=device.settings.brightness,
+                    smoothing=device.settings.smoothing,
                     state_check_interval=device.settings.state_check_interval,
                 ),
                 calibration=CalibrationSchema(**calibration_to_dict(device.calibration)),
@@ -384,7 +387,9 @@ async def get_device(
             display_index=device.settings.display_index,
             fps=device.settings.fps,
             border_width=device.settings.border_width,
+            interpolation_mode=device.settings.interpolation_mode,
             brightness=device.settings.brightness,
+            smoothing=device.settings.smoothing,
             state_check_interval=device.settings.state_check_interval,
         ),
         calibration=CalibrationSchema(**calibration_to_dict(device.calibration)),
@@ -474,6 +479,7 @@ async def update_device(
                 border_width=device.settings.border_width,
                 interpolation_mode=device.settings.interpolation_mode,
                 brightness=device.settings.brightness,
+                smoothing=device.settings.smoothing,
                 state_check_interval=device.settings.state_check_interval,
             ),
             calibration=CalibrationSchema(**calibration_to_dict(device.calibration)),
@@ -619,6 +625,7 @@ async def update_settings(
     """Update processing settings for a device.
 
     Merges with existing settings so callers can send partial updates.
+    Only fields explicitly included in the request body are applied.
     """
     try:
         # Get existing device to merge settings
@@ -627,20 +634,31 @@ async def update_settings(
             raise HTTPException(status_code=404, detail=f"Device {device_id} not found")
 
         existing = device.settings
+        sent = settings.model_fields_set  # fields the client actually sent
 
-        # Merge: use new values where provided, keep existing otherwise
+        # Merge: only override fields the client explicitly provided
         new_settings = ProcessingSettings(
-            display_index=settings.display_index,
-            fps=settings.fps,
-            border_width=settings.border_width,
-            interpolation_mode=settings.interpolation_mode,
-            brightness=settings.color_correction.brightness if settings.color_correction else existing.brightness,
-            gamma=settings.color_correction.gamma if settings.color_correction else existing.gamma,
-            saturation=settings.color_correction.saturation if settings.color_correction else existing.saturation,
-            smoothing=settings.smoothing,
-            state_check_interval=settings.state_check_interval,
+            display_index=settings.display_index if 'display_index' in sent else existing.display_index,
+            fps=settings.fps if 'fps' in sent else existing.fps,
+            border_width=settings.border_width if 'border_width' in sent else existing.border_width,
+            interpolation_mode=settings.interpolation_mode if 'interpolation_mode' in sent else existing.interpolation_mode,
+            brightness=settings.brightness if 'brightness' in sent else existing.brightness,
+            gamma=existing.gamma,
+            saturation=existing.saturation,
+            smoothing=settings.smoothing if 'smoothing' in sent else existing.smoothing,
+            state_check_interval=settings.state_check_interval if 'state_check_interval' in sent else existing.state_check_interval,
         )
 
+        # Apply color_correction fields if explicitly sent
+        if 'color_correction' in sent and settings.color_correction:
+            cc_sent = settings.color_correction.model_fields_set
+            if 'brightness' in cc_sent:
+                new_settings.brightness = settings.color_correction.brightness
+            if 'gamma' in cc_sent:
+                new_settings.gamma = settings.color_correction.gamma
+            if 'saturation' in cc_sent:
+                new_settings.saturation = settings.color_correction.saturation
+
         # Update in storage
         device = store.update_device(device_id, settings=new_settings)
 
@@ -657,6 +675,7 @@ async def update_settings(
             border_width=device.settings.border_width,
             interpolation_mode=device.settings.interpolation_mode,
             brightness=device.settings.brightness,
+            smoothing=device.settings.smoothing,
             state_check_interval=device.settings.state_check_interval,
         )